The retinoblastoma susceptibility gene (RB1) was one of the first tumor suppressor genes to be discovered and it has long served as prototype for this group of cancer-associated genes. Mutation of RB1 is the rate-limiting event in the development of the childhood cancer, retinoblastoma, and the RB1 gene product (pRb) is absent or misregulated in over 90% of human cancers. Mammalian cells contain three Rb-family members and homologous genes have been conserved during the evolution of both plants and animals. pRB binds to transcription factors and recruits chromatin modifying enzymes to promoters. pRB family members are believed to co-ordinate global changes in gene expression programs during cell cycle exit, allowing cells to switch off genes that are needed for cell proliferation and switch on genes that are needed for differentiation. How pRB family members orchestrate these changes is unclear. In the previous cycle of this grant we discovered a new activity of pRB-family proteins. We found that they physically interact with CAP-D3 a component of Condensin II complexes. Condensin complexes are conserved from yeast to humans and are required for normal chromosome structure during mitosis. Experiments using the Drosophila model system show that dCAP-D3 is extremely important for RBF1 function. We hypothesize that dCAP-D3 and Condensin II complexes have functions in addition to their well-studied roles during mitosis. We propose that Condensin II complexes are recruited to specific locations in the genome by pRB family proteins and that, at those sites, they act in conjunction with pRB family proteins to regulate levels of gene expression. To test this, we will identify genes regulated by dCAP-D3/Condensin II and RBF1. We will generate interaction-defective alleles to test the hypothesis that RBF1 and dCAP-D3 provide an element of transcriptional regulation that is distinct from its interactions with E2F. Using biochemical approaches we will purify RBF1/dCAP-D3 complexes and will examine the mechanisms underlying the functional co-operation between dCAP-D3 and RBF1. PUBLIC HEALTH RELEVANCE: Cancer is one of the leading causes of mortality in the United States. This proposal investigates the normal function of the retinoblastoma protein, pRB, one of the most famous and best characterized tumor suppressors. These experiments test the hypothesis that the localized action of pRB-family proteins at individual promoters is coupled, through an interaction with Condensin II complexes, to a more extensive organization of chromosomal domains.